Boiler

ABSTRACT

A boiler is provided with: a boiler body in which the water level inside water pipes when combustion by a burner is stopped is lower than the upper ends of the water pipes; water supply means; control means which controls the water supply means depending on the water level in the boiler body; header water level detection means which detects a water level existing inside an upper header; and a downcast pipe. The control means performs first control for controlling the water supply means so as to lower a boiler body water level when the water level detected by the header water level detection means reaches a header set water level and second control for controlling the water supply means so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control.

TECHNICAL FIELD

The present invention relates to a boiler with a small amount of water retained in a boiler body. This application claims priority to Japanese Patent Application No. 2012-017745 filed on Jan. 31, 2012, the disclosure of which is hereby incorporated by reference.

BACKGROUND ART

There is well known a boiler that is provided with: a boiler body which includes an upper header, a lower header, and a large number of water pipes to be heated by a burner, the water pipes connecting the upper header and the lower header to each other, wherein the water level inside the water pipes is lower than the upper ends of the water pipes when combustion by the burner is stopped; water supply means which supplies boiler feed water into the boiler body; and control means which controls the operation of the water supply means depending on the water level in the boiler body. Such a boiler is disclosed in Patent Document 1 or the like.

Patent Document 1: JP 2010-78204 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In a boiler retaining a relatively small amount of water as disclosed in Patent Document 1, time required for generating steam is short, and the following capacity to load change is also excellent. Therefore, such a boiler is extremely preferred in view of the operation efficiency of the boiler. However, when boiler water is highly concentrated, it is difficult to obtain a desired dryness of steam by external water level control performed by the external water level detection means connected between the upper header and the lower header. Therefore, a separator is provided as described in Patent Document 1.

The inventors of this application are engaged in the development of a so-called separatorless boiler having no separator for the purpose of reducing the number of components and reducing the cost of a product. The largest task in the development of a separatorless boiler retaining a small amount of water is how to maintain a desired dryness of steam when boiler water is highly concentrated.

As a result of studies by the inventors of this application, it has been revealed that the desired dryness of steam can be obtained by increasing the height of an upper header, for example, to 600 mm or higher in the case of a boiler having an evaporation amount of 2t. However, if the height of the upper header is increased, the amount of iron plates required for manufacturing the upper header will increase, which leads to high cost. Therefore, it is meaningless to be separatorless.

In a boiler retaining a small amount of water as described in Patent Document 1, the behavior of boiler water inside the water pipes is observed in view of the problem of overheating of the water pipes. However, since the dryness is ensured by the separator, the behavior of boiler water inside the upper header is not observed. That is, the knowledge about the behavior of boiler water inside the upper header is only enough to assume that the water level does not exist in the upper header when the boiler water is in low concentration, and a large number of air bubbles including boiler water enter the upper header when the boiler water is in high concentration. Further, when separatorless is achieved in a boiler retaining a small amount of water, in order to prevent the dryness from decreasing, it is common to perform external water level control so that the water level does not exist inside the upper header. Therefore, allowing the water level to exist inside the upper header and detecting the existing water level to thereby control the dryness is never conceivable from the common sense of the past.

In the process of the development of a separatorless boiler, the inventors of this application have observed the behavior of boiler water inside an upper header in detail and thereby acquired new knowledge such that the water level exists in the upper header when boiler body is in high concentration and, by detecting the existing water level and controlling water supply means, it is possible to maintain the level of an upper part of boiler water containing air bubbles (the upper surface of boiling boiler water) at a desired level in an internal space of the upper header to thereby obtain a desired dryness of steam.

Further, the inventors of this application have also acquired knowledge such that it is possible to ensure a predetermined circulation ratio of boiler water to thereby reduce the risk of corrosion of a boiler body by appropriately controlling timing at which the boiler body water level starts increasing.

The main problem to be solved by the present invention is to provide a boiler retaining a small amount of water capable of obtaining a desired dryness and reducing the risk of corrosion of a boiler body while suppressing the height of an upper header.

Means for Solving the Problems

The invention of the present application has been made to solve the above problem. A boiler of the invention described in claim 1 comprises: a boiler body including an upper header, a lower header, and a large number of water pipes to be heated by a burner, the water pipes connecting the upper header and the lower header to each other, wherein the water level in the water pipes when combustion by the burner is stopped is lower than the upper ends of the water pipes; water supply means supplying boiler feed water into the boiler body; control means controlling the operation of the water supply means depending on the water level in the boiler body; header water level detection means detecting a water level existing inside the upper header by boiler water inside the water pipes being pushed up by air bubbles generated by boiling of the boiler water; and downcast pipe allowing a lower part of the upper header and the lower header to communicate with each other, wherein the control means performs first control for controlling the operation of the water supply means so as to lower a boiler body water level when the water level detected by the header water level detection means reaches a header set water level and second control for controlling the operation of the water supply means so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control.

According to the invention described in claim 1, when the degree of concentration of boiler water increases and boiler water inside the water pipes is pushed up by air bubble generated by boiling of the boiler water, namely, when boiler water is in high concentration, the first control for controlling the operation of the water supply means so as to lower the boiler body water level when the header water level detection means detects a water level existing inside the upper header and the detected water level reaches the header set water level is performed. Therefore, it is possible to maintain the upper surface of boiling water inside the internal space of the upper header at a desired level. As a result, it is possible to prevent the dryness from decreasing to thereby obtain a desired dryness. In other words, the water supply control by the header water level detection means makes it possible to obtain a desired dryness without making the height of the upper header too high. Further, when boiler water is in high concentration, the second control for controlling the operation of the water supply means so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control is performed. Therefore, it is possible to maintain a predetermined circulation ratio of boiler water by circulation of boiler water through the downcast pipe.

Further, the invention described in claim 2 is characterized in that the predetermined amount is adjusted depending on any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water in claim 1.

In addition to the effects achieved by the invention described in claim 1, the invention described in claim 2 can achieve an effect such that a predetermined circulation ratio can be maintained even when any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water change.

Further, the invention described in claim 3 is characterized in that the boiler further comprises external water level detection means placed outside the boiler body, the external water level detection means communicating with an internal space of the upper header and an internal space of the lower header through communication pipes and having an electrode detecting a boiler body external water level, wherein determination of the predetermined amount by the control means is performed using the electrode in claim 1.

In addition to the effects achieved by the invention described in claim 1, the invention described in claim 3 can achieve an effect such that a predetermined circulation ratio of boiler water can be maintained by the external water level detection means.

Further, the invention described in claim 4 is characterized in that determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water in claim 3.

The invention described in claim 4 can achieve an effect such that the set time can be adjusted in response to the change of any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water.

Further, the invention described in claim 5 is characterized in that the header water level detection means includes a first electrode detecting the header set water level and a second electrode detecting a water level lower than the header set water level, and determination of the predetermined amount by the control means is performed using the second electrode in claim 1

In addition to the effects achieved by the invention described in claim 1, the invention described in claim 5 can achieve an effect such that determination of the predetermined amount by the control means can be performed without using the external water level detection means.

Further, the invention described in claim 6 is characterized in that determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the second electrode detects the non-existence of water in claim 5.

The invention described in claim 6 can achieve an effect such that the set time can be adjusted in response to the change of any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water.

Further, the invention described in claim 7 is characterized in that the header water level detection means includes an electrode detecting the header set water level, and determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water in claim 1.

In addition to the effects achieved by the invention described in claim 1, the invention described in claim 7 can achieve an effect such that the number of electrodes inside the header water level detection means can be reduced compared to the case where different electrodes are used depending on conditions such as pressure.

Effects of the Invention

The present invention makes it possible to provide a boiler retaining a small amount of water capable of obtaining a desired dryness and reducing the risk of corrosion of a boiler body while suppressing the height of an upper header.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of the longitudinal section illustrating the schematic configuration of a boiler according to Embodiment 1 of the present invention.

FIG. 2 is a diagram illustrating the schematic configuration regarding water level detection in the boiler according to Embodiment 1.

FIG. 3 is a flowchart explaining a control procedure in the boiler according to Embodiment 1.

FIG. 4 is a flowchart explaining another control procedure in the boiler according to Embodiment 1.

FIG. 5 is a diagram illustrating the schematic configuration regarding water level detection in a boiler according to Embodiment 2 of the present invention.

FIG. 6 is an explanatory diagram of the longitudinal section illustrating the schematic configuration of a boiler according to Embodiment 3 of the present invention.

FIG. 7 is a flowchart explaining a control procedure in the boiler according to Embodiment 3.

FIG. 8 is an explanatory diagram of the longitudinal section illustrating the schematic configuration of a boiler according to Embodiment 4 of the present invention.

FIG. 9 is a flowchart explaining a control procedure in the boiler according to Embodiment 4.

FIG. 10 is a flowchart explaining a control procedure in a boiler according to Embodiment 5 of the present invention.

FIG. 11 is a flowchart explaining another control procedure in the boiler according to Embodiment 5.

FIG. 12 is an explanatory diagram of the longitudinal section illustrating the schematic configuration of a boiler according to Embodiment 6 of the present invention.

EXPLANATION OF REFERENCE NUMERALS

-   10 boiler -   20 boiler body -   22 lower header -   23 water pipe -   24 upper header -   50 external water level detection device (external water level     detection means) -   52S, 52M first electrode (electrode) -   60 header water level detection device (header water level detection     means) -   62E sixth electrode (first electrode) -   62F seventh electrode (second electrode) -   70 water supply pump (water supply means) -   84 downcast pipe -   87 a, 87 b communication pipe -   88 a, 88 b communication pipe -   100 controller (control means)

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Next, an embodiment of a boiler of the present invention will be described. The embodiment of the present invention is preferably applied to a separatorless boiler retraining a small amount of water.

Before describing the embodiment of the present invention, terms used in the present application will be described. Retaining a small amount of water means that the water level in a water pipe of a boiler body is lower than the upper end of the water pipe when combustion by a burner is stopped. Further, in this definition, “when combustion by a burner is stopped” can be replaced with “when steam starts being generated”, and “lower than the upper end of the water pipe” can be replaced with “on the intermediate part of the water pipe”. Further, a separatorless boiler is a boiler that is not provided with a separator (which is also referred to as a gas-liquid separator or an air-water separator). In this regard, a boiler in which a baffle plate is provided above the upper ends of a plurality of water pipes located near an opening of a steam outlet pipe of an upper header does not have an independent space as a separator, and is therefore included in a separatorless boiler.

EMBODIMENT

The embodiment of the present invention will be specifically described. A boiler of this embodiment is provided with: a boiler body which includes an upper header, a lower header, and a large number of water pipes to be heated by a burner, the water pipes connecting the upper header and the lower header to each other, wherein the water level inside the water pipes when combustion by the burner is stopped is lower than the upper ends of the water pipes; water supply means which supplies boiler feed water into the boiler body; and control means which controls the operation of the water supply means depending on the water level in the boiler body. Since the water level inside the water pipes is lower than the upper ends of the water pipes when combustion by the burner is stopped, the boiler can be referred to as a boiler retaining a small amount of water.

Feature of Embodiment

A feature of the embodiment is a configuration that is provided with header water level detection means which detects a water level that exists inside the upper header by boiler water inside the water pipes being pushed up by air bubbles generated by boiling of the boiler water and a downcast pipe which allows a lower part of the upper header and the lower header to communicate with each other, wherein the control means performs first control for controlling the operation of the water supply means so as to lower the boiler body water level when the water level detected by the header water level detection means reaches a header set water level and second control for controlling the operation of the water supply means so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control. The present invention is preferably applied to a separatorless boiler that is not provided with a separator. However, the present invention may be applied to a boiler that has the above feature and is provided with a separator smaller (with smaller quantity) than a conventional one.

The header set water level can be referred to as a high concentration time dryness limit water level as an upper limit water level for maintaining a predetermined dryness (0.98 or higher, for example) at the time of high concentration. Further, “controlling the operation of the water supply means so as to lower (or raise) the boiler body water level” means turning OFF (or ON) the water supply means when the water supply means controls the water supply amount by ON-OFF thereof, and means reducing (or increasing) the amount of water supplied by the water supply means when the water supply means controls the water supply amount in a multistage manner or proportionally. Further, “the water level reaches a set water level” includes both of the case where the water level becomes equal to or higher than the set water level and the case where the water level becomes higher than the set water level. Further, “so as to lower (raise) the boiler body water level” can be replaced with “so as to lower (raise) the boiler body external water level”.

In the embodiment having such a configuration, when the degree of concentration of boiler water becomes high (high concentration) along with the operation of the boiler, boiler water inside the water pipes is pushed up by air bubbles generated by boiling of the boiler water, so that a water level that can be detected by the header water level detection means exists inside the upper header. Then, when the header set water level is detected by the header water level detection means, the operation of the water supply means is controlled so as to lower the boiler body water level. Therefore, it is possible to maintain an upper part of boiler water containing air bubbles (the upper surface of boiling water) at a desired level in an internal space of the upper header. As a result, it is possible to prevent the dryness from decreasing to obtain a predetermined dryness (0.98 or higher, for example). The degree of concentration of boiler water can be divided into low concentration and high concentration with, for example, 300 to 400 mS/m (not limited to these values) as a boundary.

Further, when the water level control by the header water level detection means is not performed, it is necessary to set the height of the upper header to 600 mm or higher, for example, in a boiler having an evaporation amount of 2t/h. However, by virtue of the water level control by the header water level detection means of this embodiment, it is possible to prevent the height of the upper header from increasing (for example, set to preferably 400 mm or less, and more preferably 300 mm or less).

Further, when the circulation ratio is low, the pH of a part to which water with a low pH is supplied becomes low, and the risk of corrosion thereby increases. However, by increasing the circulation ratio, supplied water with a low pH and boiler water with a high pH which is supplied to the lower header through the downcast pipe can be mixed to maintain the inside of the boiler body at an appropriate pH, thereby making it possible to reduce the risk of corrosion. In addition, supplied water with a low temperature and boiler water with a high temperature can be mixed to achieve high temperature distribution in the entire boiler water, thereby making it possible to reduce the risk of corrosion caused by dissolved oxygen.

The circulation ratio is defined as (evaporation amount+(lowdown water amount)/(water supply amount). The evaporation amount is the amount of steam that flows out of the upper header. The flowdown water amount is the amount of boiler water (circulation water) that flows down through the downcast pipe. When evaporation amount=water supply amount, the circulation ratio increases as the downflow water amount increases.

(Mode for Achieving Second Control)

The second control in the feature of the embodiment described above is a configuration aiming at increasing the circulation ratio of boiler water. In the second control, timing of controlling the water supply means so as to raise the boiler body water level, that is, timing of determining that the amount of drop in the boiler body water level reaches a predetermined amount after the first control is set at timing when the water level is lowered up to a target circulation ratio set water level which is set to be higher than an overheating limit water level for preventing overheating.

The target circulation ratio set water level is set to obtain a target circulation ratio. In a once-through boiler, the circulation ratio is 2 or less. On the other hand, in the boiler of the present embodiment, the target circulation ratio is appropriately set within the following range. A lower limit of the target circulation ratio is set on the basis of the risk of corrosion relating to the pH and the concentration of dissolved oxygen inside the boiler body. Further, since the dryness limit water level becomes difficult to control when the circulation ratio is made too large, an upper limit of the target circulation ratio is set on the basis of the limit of dryness. In the boiler of the present embodiment, for example, if the circulation ratio is set to 2 or less with respect to water having high corrosiveness as in the case of a once-through boiler, the risk of corrosion increases. Therefore, it is desired to set the circulation ratio to a value as large as possible within the range larger than 2, but equal to or less than the upper limit.

In the present embodiment, preferably, the predetermine amount, that is, the target circulation ratio set water level is adjusted depending on any one or more (two or three) of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water (water inside the boiler body). An increase in a value of the pressure inside the boiler body is advantageous in maintaining the dryness, but disadvantageous in maintaining a predetermined circulation ratio and preventing overheating. On the other hand, an increase in a value of each of the temperature of supplied water and the degree of concentration of boiler water is disadvantageous in maintaining the dryness, but advantageous in maintaining a predetermined circulation ratio and preventing overheating. Therefore, the target circulation ratio set water level is made higher as the pressure inside the boiler body increases. On the other hand, the target circulation ratio set water level is made lower as the temperature of water supplied to the boiler body or the degree of concentration of boiler water increases. In other words, the target circulation ratio set water level is controlled to be higher as the pressure inside the boiler body increases or the temperature of supplied water or the degree of concentration of boiler water drops (decreases).

The control can be achieved in the following manner. For example, when the adjustment is performed on the basis of both of the pressure inside the boiler body and the temperature of supplied water, a table in which the target circulation ratio set water level which changes in response to the change of the pressure inside the boiler body and the temperature of supplied water is stored in a memory. Then, a target circulation ratio set water level that corresponds to a detected boiler body pressure and a detected supplied water temperature is read out, and water level control on the basis of the read-out target circulation ratio set water level is performed. The degree of concentration of boiler water can be detected in a lower part of the downcast pipe, for example.

Such a configuration makes it possible to maintain a predetermined circulation ratio even when any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water change.

More specifically, the second control of the present embodiment can be achieved by the following first to fourth modes.

The first mode is provided with external water level detection means which is placed outside the boiler body, communicates with an internal space of the upper header and an internal space of the lower header through communication pipes, and has an electrode which detects a boiler body external water level. Further, determination of the predetermined amount by the control means is performed using the electrode.

In the first mode, when the boiler body external water level is lowered up to a predetermined boiler body external set water level (target circulation ratio set water level), the electrode detects the non-existence of water. Then, the control means thereby determines that the amount of drop in the boiler body water level reaches the predetermined amount, and performs the second control for controlling the operation of the water supply means so as to raise the boiler body water level to thereby ensure a predetermined circulation ratio.

In the first mode, preferably, determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water. Further, preferably, the set time is adjusted depending on any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water. Specifically, the set time is made shorter as the pressure inside the boiler body increases, or the temperature of supplied water or the degree of concentration of boiler water drops.

In the first mode, such a configuration makes it possible to maintain a predetermined circulation ratio even when any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water change.

In the second mode, the header water level detection means is provided with a first electrode which detects the header set water level and a second electrode which detects a water level lower than the header set water level, and determination of the predetermined amount by the control means is performed using the second electrode.

In the second mode, when the header water level is lowered up to a predetermined water level that is lower than the header set water level and the second electrode thereby detects the non-existence of water, the control means determines that the amount of drop in the boiler body water level reaches the predetermined amount and performs the second control for controlling the operation of the water supply means so as to raise the boiler body water level to thereby ensure a predetermined circulation ratio.

Also in the second mode, preferably, determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the second electrode detects the non-existence of water. Further, preferably, the set time is adjusted depending on any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water. Specifically, the set time is made shorter as the pressure inside the boiler body increases, or the temperature of supplied water or the degree of concentration of boiler water drops.

Also in the second mode, such a configuration makes it possible to maintain a predetermined circulation ratio even when any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water change.

In the third mode, the header water level detection means is provided with an electrode which detects the header set water level, and determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water.

In the third mode, when the set time elapses after the electrode stop detecting the header set water level, the control means determines that the amount of drop in the boiler body water level reaches the predetermined amount, and performs the second control for controlling the operation of the water supply means so as to raise the boiler body water level to thereby ensure a predetermined circulation ratio.

Also in the third mode, preferably, the set time is adjusted depending on any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water. Specifically, the set time is made shorter as the pressure inside the boiler body increases, or the temperature of supplied water drops.

Also in the third mode, such a configuration makes it possible to maintain a predetermined circulation ratio even when any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water change.

In the fourth mode, the water level detection by the external water level detection means or the header water level detection means is performed using a water level sensor (a differential pressure sensor, for example) which can change the set water level without using an electrode. Further, the target circulation ratio set water level itself is changed, and the amount of drop in the water level is determined to reach the predetermined amount when the water level is lowered up to the changed target circulation ratio set water level.

(Additional Feature of First Control)

The embodiment described above can further be provided with one or more in combination of the following additional first to fourth features. Note that “additional” means that the additional feature solves a problem that is different from the problem solved by the main feature.

The first feature is a configuration aiming at allowing the water level inside the upper header by the header water level detection means to stably exist to thereby perform stable water level control using the header set water level. More specifically, the first feature is that the header water level detection means includes a water level control chamber which has communication holes respectively communicating with an upper part and a lower part of the upper header and an electrode which detects the header set water level inside the water level control chamber. The upper part of the upper header is desirably located at a position as high as possible inside the upper header, and the lower part of the upper header is desirably located at a position as low as possible inside the upper header. Further, “water level control chamber” can be referred to as “water level control container”. The shape of the water level control chamber is preferably a tubular shape, but not limited thereto.

According to the first feature, the pressure in the header water level detection means can be made equal to the pressure inside the internal space of the upper header through the upper communication hole and the lower communication hole. Further, it is possible to allow boiler water to flow into the water level control chamber through the lower communication hole. Therefore, the water level of boiler water inside the upper header can be made substantially equal to the water level of boiler water detected in the water level control chamber. Further, although the upper surface of boiling water in the upper header when boiler water is in high concentration is unstable, the first feature makes it possible to form a stable water level inside the water level control chamber. As a result, it is possible to stably perform detection of the header set water level by the header water level detection means.

A mode for achieving the first feature includes the following two modes (first and second modes). In the first mode, the water level control chamber of the header water level detection means is placed inside the upper header, and communicates with the upper part and the lower part of the upper header through the communication holes. In the second mode, the water level control chamber of the header water level detection means is placed outside the upper header, and the upper part and the lower part of the upper header communicate with the respective communication holes through communication pipes.

The second mode in which the header water level detection means is provided outside the upper header requires the communication pipes for connecting the water level control chamber and the upper header to each other. On the other hand, in the first mode, no communication pipe is required, and it is not necessary to provide a space in which the header water level detection means is to be placed separately from the upper header. Therefore, it is possible to reduce a placement space compared to the second mode.

Further, the first mode in which the header water level detection means is provided inside the upper header involves a slightly unstable water surface caused by air bubbles flowing into the water level control chamber through the communication holes. On the other hand, the second mode makes it possible to allow a more stable upper header water level to exist inside the water level control chamber compared to the first mode. As a result, it is possible to perform stable water level control.

The second feature is a mode regarding a method for switching control between the dryness limit water level control by the external water level detection means at the time of low concentration (external water level control) and the dryness limit water level control by the header water level detection means at the time of high concentration (header water level control). The mode of the second feature includes the following two modes (first and second modes).

In the first mode of the second feature, the water supply means is controlled so as to lower the boiler body water level on the basis of OR condition between a condition in which the external water level detection means detects the boiler body external set water level and a condition in which the header water level detection means detects the header set water level (first control condition). In this mode, switching between the external water level control and the header water level control is automatically performed without determining the concentration of boiler water.

In the second mode of the second feature, switching between the external water level control and the header water level control is performed by determining the degree of concentration of boiler water. Although the second mode includes the following two aspects (first and second aspects), the second mode also includes determination methods of the degree of concentration other than these aspects. In the first aspect, the degree of concentration is determined on the basis of whether a combustion time of the burner after discharging boiler water existing inside the boiler body to the outside reaches a combustion time threshold. In the second aspect, the degree of concentration is determined by electric conductivity measuring means which measures the electric conductivity of boiler water inside the boiler body.

Specifically, the first aspect is configured in the following manner. When the combustion time of the burner does not reach the combustion time threshold, the control means controls the water supply means so as to lower the boiler body water level when the external water level detection means detects the boiler body external set water level. On the other hand, when the combustion time of the burner has reached the combustion time threshold, the control means controls the operation of the water supply means so as to lower the boiler body water level when the header water level detection means detects the header set water level.

Further, specifically, the second aspect is configured in the following manner. When the electric conductivity of boiler water measured by the electric conductivity measuring means does not reach an electric conductivity threshold, the control means controls the water supply means so as to lower the boiler body water level when the external water level detection means detects the boiler body external set water level. On the other hand, when the electric conductivity of boiler water measured by the electric conductivity measuring means has reached the electric conductivity threshold, the control means controls the operation of the water supply means so as to lower the boiler body water level when the header water level detection means detects the header set water level.

The third feature is that the control means controls the water supply means so as to raise the boiler body water level when the external water level detection means stops detecting the first set water level and the header water level detection means stops detecting the second set water level (second control condition).

The configuration that increases the water supply amount on the basis of the second control condition achieves the following effects. When the external water level detection means stops detecting the first set water level and drives the water supply means while the header water level detection means is detecting the header set water level, that is, when the water supply amount is increased by the water supply means in the state where a water level that is higher than the dryness limit water level exists in the upper header, steam having low dryness may disadvantageously flow out of the upper header. However, the configuration that increases the water supply amount on the basis of the second control condition can solve this problem to thereby prevent the dryness from decreasing. Further, while the header water level detection means is detecting the header set water level, even if the external water level detection means stops detecting the boiler body external set water level, the water supply amount is not increased by the water supply means. Therefore, the boiler body external water level can be lowered with a low risk of overheating the water pipes.

Further, in the configuration that increases the water supply amount on the basis of the second control condition, preferably, the external water level detection means includes a water level control chamber which communicates with the upper header and the lower header, a first electrode which detects the first boiler body external set water level inside the water level control chamber, and a second electrode which detects a second boiler body external set water level which is lower than the first boiler body external water set level inside the water level control chamber. In addition, when the second electrode stops detecting the second boiler body external set water level, the control means controls the water supply means so as to raise the boiler body water level.

Such a configuration achieves the following effect. When a state that does not satisfy the second control condition continues for a long period of time, the boiler body water level is excessively lowered while the pressure inside the boiler body is excessively increased. However, in this configuration, the water supply means is controlled so as to raise the boiler body water level when the second electrode stops detecting the second boiler body external set water level. Therefore, it is possible to prevent the pressure inside the boiler body from excessively increasing. As a result, it is possible to reduce pressure fluctuation inside the boiler body. It would appear that the increase in the pressure inside the boiler body due to the decrease in the water supply amount is caused by a decrease in the cooling amount of boiler water by the water supply.

The fourth feature is a configuration aiming at preventing a decrease in the dryness caused by that, when the combustion amount of the burner changes in a stepwise manner, different boiling states of boiler water are caused even with the same degree of concentration (boiling becomes stronger as the combustion amount increases). In the fourth feature, the header water level detection means includes a first electrode which detects a plurality of different header set water levels corresponding to different combustion amounts, and the control means controls the operation of the water supply means so as to lower the boiler body water level when the header set water levels corresponding to different combustion amounts are detected. For example, in a boiler in which the combustion amount is switched between high combustion and low combustion, the boiler is provided with a high combustion first electrode which detects a high combustion header set water level and a low combustion first electrode which detects a low combustion second set water level that is higher than the high combustion header set water level. Further, the operation of the water supply means is controlled so as to lower the boiler body water level when the high combustion first electrode detects the high combustion second set water level at the time of high combustion and when the low combustion first electrode detects the low combustion header set water level at the time of low combustion.

Here, the components constituting the boiler of the embodiment of the present invention described above will be described. Description of components which have been already described above will be omitted. The boiler body is not limited to a so-called round boiler body in which a large number of water pipes are annularly arranged, and includes a so-called square boiler body in which the flowing direction of combustion gas is linearly formed and a plurality of water pipes are arranged along the flow of the combustion gas. The flow of combustion gas (including gas containing combustion flame and exhausted gas) in a round boiler body is not limited to flow in the horizontal direction such as ω flow, and includes flow in the up-down direction. Further, the shape of the upper header and the lower header is not limited to a donut shape and an annular shape. Further, the burner and the water supply means are not limited to a particular structure and type.

Embodiment 1

Next, Embodiment 1 of the boiler of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram of the longitudinal section illustrating the schematic configuration of a boiler according to Embodiment 1 of the present invention. FIG. 2 is a diagram illustrating the schematic configuration regarding water level detection in the boiler according to Embodiment 1. FIG. 3 is a flowchart explaining a control procedure in the boiler according to Embodiment 1. FIG. 4 is a flowchart explaining another control procedure in the boiler according to Embodiment 1.

Configuration of Embodiment 1

As illustrated in FIG. 1, a boiler 10 is provided with a boiler body 20, a burner 30, a fireproof material 40, an external water level detection device 50 as the external water level detection means, a header water level detection device 60 as the header water level detection means, a water supply pump 70 as the water supply means, and a controller 100 as the control means. In these components, the boiler body 20 includes, as main components, a boiler body cover 21, a lower header 22, water pipes 23, and an upper header 24. The boiler 10 can be controlled at three states including stop, low combustion, and high combustion by adjusting the combustion amount of the burner 30 at three stages including zero. The low combustion operation and the high combustion operation can be respectively referred to as low load operation and high load operation.

The boiler body cover 21 is made by forming a plate material in a circular chamber shape, and covers the water pipes 23 to isolate the water pipes 23 from the outside. The lower header 22 is formed in a hollow ring shape. A water supply pipe 80 is connected to the lower header 22 so that boiler feed water can be supplied to the lower header 22 through the water supply pipe 80. Further, a main drain pipe 81 is also connected to the lower header 22. A main drain valve 82 is provided in the main drain pipe 81. By opening the main drain valve 82, boiler water inside the lower header 22 can be discharged (blown) to the outside. In this specification, “boiler water” indicates boiler feed water that has been introduced into the boiler body 20 (the lower header 22, the water pipes 23 and the like).

The lower ends of the water pipes 23 are connected to the lower header 22. Each of the water pipes 23 is a tubular member which communicates with each of the lower header 22 and the upper header 24 to store boiler water therein. By heating the water pipes 23 by combustion by the burner 30, boiler water inside the water pipes 23 boils. Each of the water pipes 23 extends along the vertical direction. In Embodiment 1, a large number of water pipes 23 are arranged. In the arrangement of the water pipes 23, there are an outer water pipe row 23A which is arranged along the circumference of a first circle that is separated from the center in the radial direction of the boiler body 20 by a first radius and an inner water pipe row 23B which is arranged along the circumference of a second circle that is separated from the center in the radial direction of the boiler body 20 by a second radius that is smaller than the first radius. However, the water pipes 23 are not limited to the configuration having two rows including the outer water pipe row 23A and the inner water pipe row 23B, and may have only one row or may also have three or more rows.

The upper end of each of the water pipes 23 is connected to the upper header 24. The upper header 24 is a portion into which boiler water inside the water pipes 23 is pushed up by air bubbles generated by boiling of the boiler water when the boiler water is concentrated. In other words, the upper header 24 is a portion into which boiler water whose volume has expanded due to the existence of steam boiling in the water pipes 23 and air bubbles during the boiling is introduced. The upper header 24 is also formed in a hollow ring shape as with the lower header 22 described above. In the following description, an internal space forming the hollow of the upper header 24 is referred to as an internal space 24A. The internal space 24A communicates with openings 23 s of the respective water pipes 23 and also communicates with an opening 83 s of a steam outlet pipe 83. Further, in the following description, an internal space forming the hollow of the lower header 22 is referred to as an internal space 22A.

In Embodiment 1, the upper header 24 is higher (larger in the vertical direction) than a conventional upper header 24. For example, in a boiler having an evaporation amount of 2t/h, the height of the upper header 24 in Embodiment 1 is 300 mm while the height of the conventional upper header 24 is 150 mm. The boiler 10 in Embodiment 1 is a separatorless boiler that is not provided with a separator. However, even in a separatorless boiler, a predetermined dryness can be obtained while preventing the height of a separator from increasing by dryness limit water level control by the header water level detection device 60 (described below).

Further, a baffle plate 25 is provided inside the upper header 24. The baffle plate 25 functions to prevent air bubbles in boiling boiler water in water pipes 23 located near the steam outlet pipe 83 from directly flowing out through the steam outlet pipe 83. In Embodiment 1, the baffle plate 25 is provided so as to cover a space above water pipes 23 located within the range that is a half of the upper header 24 (half of the donut shape) at the side near the opening of the steam outlet pipe 83. The baffle plate 25 can be omitted, or the area of the baffle plate 25 (the area covering the large number of water pipes 23) can be increased or reduced as needed.

Increasing the height of the internal space 24A of the upper header 24 means increasing the distance of a flow path between the opening 23 s of each of the water pipes 23 located on the bottom surface of the upper header 24 and the opening 83 s of the steam outlet pipe 83 which is connected to a top plate 24B of the upper header 24. The extension of the distance of the flow path between the two openings (between the opening 23 s and the opening 83 s) leads to achieving a function of air-water separation. However, the height of the internal space 24A will disadvantageously be 600 mm or higher in a boiler having an evaporation amount of 2t/h merely by increasing the height. Therefore, in Embodiment 1, the height of the internal space 24A, that is, the height of the upper header 24 is suppressed by overheating limit water level control (described below).

As illustrated in FIG. 1, a lower part of the internal space 24A of the upper header 24 communicates with the upper end of a downcast pipe 84. The downcast pipe 84 returns boiler water existing in the internal space 24A (the boiler water is often concentrated) to the lower header 22. Therefore, the lower end of the downcast pipe 84 communicates with the internal space 22A of the lower header 22. A concentrated water drain pipe 85 is connected to the downcast pipe 84. A concentrated water drain valve 86 is provided in the concentrated water drain pipe 85. Concentrated boiler water can be discharged to the outside by opening the concentrated water drain valve 86.

The burner 30 is provided in an upper part of the boiler body 20. The burner 30 is located inside a ring hole (reference numeral of which is omitted) of the ring-shaped upper header 24. The burner 30 forms flame on an inner part of the inside of the boiler body cover 21, the inner part being surrounded by the water pipes 23 (hereinbelow, referred to as a combustion chamber 21A). Fuel and combustion air are supplied to the burner 30 for combustion inside the combustion chamber 21A. The fuel is supplied through a fuel pipe 32 which is provided with a fuel valve 31. The air is supplied by a blower 33.

The fireproof material 40 is provided on a lower part of the boiler body 20. The fireproof material 40 blocks the lower part of the boiler body 20 (the part on which the lower parts of the water pipes 23 are located (narrowed parts of the water pipes) and a part located on the inner side with respect thereto). Accordingly, the combustion chamber 21A is formed on the central side in the radial direction with respect to the inner water pipe row 23B. Further, another fireproof material 40 is provided on an upper part of the inside of the boiler body cover 21 at a position corresponding to the upper parts of the water pipes 23 (narrowed parts of the water pipes).

Next, the external water level detection device 50 illustrated in FIGS. 1 and 2 detects the water level of boiler water introduced into the boiler body 20 (the lower header 22, the water pipes 23 and the like) (referred to as a boiler body water level) as an external water level. The external water level detection device 50 is provided with a water level control tube 51 as the water level control chamber and a plurality of bar-shaped electrodes 52.

The water level control tube 51 is formed in a generally cylindrical shape both ends of which are closed using conductible metal. The upper end of the water level control tube 51 and the lower end of a communication pipe 87 a are connected to each other. The upper end of the communication pipe 87 a is connected to the upper header 24. Further, the lower end of the water level control tube 51 and the upper end of a communication pipe 87 b are connected to each other. The lower end of the communication pipe 87 b is connected to the lower header 22. Accordingly, the upper end and the lower end of the water level control tube 51 communicate with the water pipes 23 respectively through the upper header 24 and the lower header 22. Therefore, the water level control tube 51 achieves a boiler body water level corresponding to boiler water introduced into the water pipes 23 (boiler body external water level) inside the water level control tube 51.

The electrodes 52 include a low combustion first electrode 52S, a high combustion first electrode 52M, and a third electrode 52L which detect respective different water levels in the water level control tube 51. The low combustion first electrode 52S detects a low combustion boiler body external set water level H1S as a dryness limit water level of boiler water in low concentration (low concentration time dryness limit water level) during low combustion. The low combustion boiler body external set water level H1S is set to be higher than an overheating-degree limit water level of boiler water in low concentration during low combustion. Also, the low combustion boiler body external set water level H1S is a target circulation ratio set water level of boiler water in low concentration during low combustion. Further, the high combustion first electrode 52M detects a high combustion boiler body external set water level H1M (which is lower than the low combustion boiler body external set level H1S as a dryness limit water level of boiler water in low concentration (low concentration time dryness limit water level) during high combustion. The high combustion boiler body external set water level H1M is set to be higher than an overheating-degree limit water level of boiler water in low concentration during high combustion. Also, the high combustion boiler body external set water level H1M is a target circulation ratio set water level of boiler water in high concentration during high combustion (which is substantially equal to the target circulation ratio set water level of boiler water in low concentration). Further, the third electrode 52L detects a second boiler body external set water level H3L which is lower than the high combustion boiler body external set water level H1M for preventing overheating of the water pipes. Each of the set water levels described above is previously set by performing experiments.

The header water level detection device 60 detects the water level of boiler water in high concentration in the internal space 24A of the upper header 24. A state of the water level of boiler water in high concentration detected by the header water level detection device 60 is illustrated in FIG. 2. Referring to FIG. 2, when boiler water inside the water pipes 23 boils, a large number of air bubbles K are generated inside the water pipes 23. In an upper part of boiler water containing the air bubbles K (the upper surface of boiling boiler water), the air bubbles K are successively exposed and scatter. In such a state, some of the air bubbles K enter the internal space 24A. Inside the internal space 24A, the air bubbles K burst, so that an inner wall surface of the internal space 24A and an inner surface of the upper part of each of the water pipes 23 are covered by films of liquid droplets generated from the thus burst air bubbles (water films). At this point, a water level having an unstable boiling upper surface exists in the lower part of the internal space 24A. On the other hand, some of the air bubbles K enter the water level control tube 61 through a communication pipe 88 b (described below), and liquid droplets generated from burst air bubbles K also flow into the water level control tube 61 through the communication tube 88 b (described below). These air bubbles K and liquid droplets can be detected as the amount of water and appear as the water level inside the water level control tube 61. Therefore, the header water level detection device 60 detects the water level that appears inside the water level control tube 61.

The header water level detection device 60 has the same configuration as the external water level detection device 50. More specifically, the header water level detection device 60 is provided with a water level control tube 61 as the water level control chamber and a plurality of electrodes 62. A communication hole 64 a on the upper end of the water level control tube 61 and the lower end of a communication pipe 88 a are connected to each other. The upper end of the communication pipe 88 a is connected to the upper part of the upper header 24. Further, a communication hole 64 b on the lower end of the water level control tube 61 and the upper end of the communication pipe 88 b are connected to each other. The lower end of the communication pipe 88 b is connected to the lower part of the lower header 22.

The electrodes 62 include a low combustion second electrode 62S and a high combustion second electrode 62M which detect respective different water levels in the water level control tube 61. The low combustion second electrode 62S detects a low combustion second set water level H2S as a dryness limit water level of boiler water in high concentration (high concentration time dryness limit water level) during low combustion. Further, the high combustion second electrode 62M detects a high combustion header set water level H2M (which is lower than the low combustion second set level H2S) as a dryness limit water level of boiler water in high concentration (high concentration time dryness limit water level) during high combustion. The low combustion second electrode 62S and the high combustion second electrode 62M can be provided in respective different water control tubes.

Referring to FIG. 1, the boiler body 20 is provided with a fourth electrode 62D which detects the water level inside the water pipes 23. The fourth electrode 62D detects a fourth set water level H4D as the water level at the time of staring boiling in the boiler 10 (not limited to combustion operation of the boiler 10 in a cold state). The controller 100 operates an operation starting switch (not illustrated) to thereby drive the water supply pump 70 to supply boiler water from the lower header 22 into the water pipes 23. When the fourth electrode 62D detects the fourth set water level H4D, the controller 100 stops the water supply pump 70 and shifts to the low combustion operation.

With the configuration of the boiling starting time water level control, since the water level in the water pipes 23 rises up to the fourth set water level H4D, it is possible to prevent overheating of the water pipes at the time of starting boiling. Further, if boiling is started in the state where boiler water enters the upper header 24 beyond the fourth set water level H4D, a large amount of boiler water may disadvantageously flow out through the steam outlet pipe 83. However, the configuration of the boiling starting time water level control can prevent such a problem.

Further, the water supply pump 70 is an ON-OFF type pump, and connected to the lower header 22 through the water supply pipe 80. When the water supply pump 70 is actuated by the control by the controller 100, the water supply pump 70 starts the supply of boiler feed water to the lower header 22. A check valve 89 is provided on the water supply pipe 80 between the lower header 22 and the water supply pump 70 to prevent boiler water from flowing back toward the water supply pump 70 from the lower header 22.

Detection signals from various sensors such as the electrodes of the external water level detection device 50, the electrodes of the header water level detection device 60, and the fourth electrode are input to the controller 100. The controller 100 controls the operations of the drive parts such as the burner 30 and the water supply pump 70 in response to the input detection signals. The control means 100 performs a combustion control procedure and a water level control procedure by various programs stored in a storage device (not illustrated). The water level control procedure includes a dryness maintaining control procedure and a circulation ratio ensuring control procedure. The water level control procedures during high combustion and low combustion are respectively illustrated in FIG. 3 and FIG. 4. In FIGS. 3 and 4, the water level control procedure at the time of starting boiling by the fourth electrode described above is omitted.

Operation in Embodiment 1

The operation of the water level control in the boiler 10 having the above configuration will be described below with reference to FIGS. 1 to 4.

First, the water level control during the high combustion operation of the boiler 10 will be described with reference to FIG. 3. In step S1 (hereinbelow, “step SN” is just referred to as “SN”), the controller 100 determines whether the high combustion second electrode 62M detects the non-existence of water (does not detect the water level) and the high combustion first electrode 52M detects the non-existence of water (does not detect the water level), that is, whether both of the conditions are satisfied (AND condition is satisfied) (second control condition).

At this point, when boiler water is in a low concentration state, the water level is not detected by the high combustion second electrode 62M of the header water level detection device 60, and the boiler body water level is detected by the external water level detection device 50. When the water level in the water level control tube 51 is less than H1M, YES is determined in S1, and the step proceeds to S2 to turn ON the water supply pump 70. Accordingly, boiler feed water is supplied to the lower header 22, and the water level inside the boiler body 20 thereby rises.

Then, in S3, it is determined whether the high combustion second electrode 62M detects the existence of water (detects the water level) or the high combustion first electrode 52M detects the existence of water (detects the water level), that is, whether either one of the conditions is satisfied (OR condition is satisfied, first control condition). At this point, when the boiler body water level rises, so that the water level in the water level control tube 51 becomes H1M or higher and the high combustion first electrode 52M thereby detects the existence of water, YES is determined in S3, and the step proceeds to S4 to turn OFF the water supply pump 70. In this manner, when boiler water is in low concentration during the high combustion operation, water level control that maintains a predetermined dryness and does not cause overheating of the water pipes is performed using the high combustion first electrode 52M. Further, the time between when the high combustion first electrode 52M detects the existence of water and when the water supply pump 70 is turned OFF and the time between when the high combustion first electrode 52M detects the non-existence of water and when the water supply pump 70 is turned ON are adjustable.

After the concentration of boiler water makes progress and the boiler water becomes a high concentration state by the operation of the boiler 10, the high combustion second electrode 62M of the header water level detection device 60 substantially constantly detects the water level. As a result, even if the high combustion first electrode 52M detects the non-existence of water, when the high combustion second electrode 62M detects the existence of water, YES is determined in S3, and the step proceeds to S4 to turn OFF the water supply pump 70. As a result, the water level inside the boiler body 20 is lowered.

Then, the step proceeds to S5 to determine whether the third electrode 52L detects the non-existence of water. When NO is determined, the step moves back to S1. In S1, when both of the high combustion first electrode 52M and the high combustion second electrode 62M detect the non-existence of water (second control condition), the step proceeds to S2 to turn ON the water supply pump 70.

The configuration that turns ON the water supply pump 70 when the second control condition is satisfied in S1 achieves the following effect. Specifically, when the high combustion first electrode 52M detects the non-existence of water and the water supply pump 70 is thereby driven while the high combustion second electrode 62M is detecting the existence of water, the water supply amount is increased by the water supply pump 70 in the state where a water level that is higher than the dryness limit water level exists in the upper header 24. As a result, steam having low dryness disadvantageously flows out of the upper header 24. However, the configuration that increases the water supply amount on the basis of the second control condition can solve this problem and prevent the dryness from decreasing. Further, while the high combustion second electrode 62M is detecting the existence of water, even if the high combustion first electrode 52M detects the non-existence of water, the water supply amount is not increased by the water supply pump 70. Therefore, the water level inside the boiler body 20 can be lowered with a low risk of overheating of the water pipes.

Further, the high combustion second electrode 62M may continuously detect the non-existence of water, and YES may be thereby continuously determined in S1. In such a case, YES is determined in S5 since the third electrode 52L detects the non-existence of water, and the step proceeds to S6 to turn ON the water supply pump 70. As a result, it is possible to prevent the pressure inside the boiler body 20 from increasing due to an OFF state of the water supply pump 70 continuing for a long period of time, and thereby reduce pressure fluctuation inside the boiler body 20.

In this manner, the water level inside the upper header 24 is controlled to be equal to or lower than the dryness limit water level using the high combustion second electrode 62M when boiler water is in high concentration during the high combustion operation. Therefore, water level control that maintains a predetermined dryness and does not cause overheating of the water pipes is performed even in a high combustion and high concentration state.

Further, when boiler water is in high concentration during the high combustion operation, boiler water existing in the lower part of the upper header 24 flows down through the downcast pipe 84, and returns into the lower header 22. As a result, it is possible to prevent an excessive water level from existing inside the upper header 24 and ensure a predetermined circulation ratio of boiler water.

The circulation ratio increases as raising the timing of turning ON the water supply pump 70, that is, the water level detected by the high combustion first electrode 52M. When the water level detected by the high combustion first electrode 52M is made too high, the dryness is reduced. In Embodiment 1, the water level detected by the high combustion first electrode 52M is set to be the target circulation ratio set water level which is higher than the overheating limit water level for preventing overheating. Therefore, the water level control can be performed with a predetermined circulation ratio. As a result, supplied water with a low pH and boiler water with a high pH which is supplied to the lower header through the downcast pipe can be moderately mixed to maintain the inside of the boiler body at an appropriate pH, thereby making it possible to reduce the risk of corrosion. In addition, supplied water with a low temperature and boiler water with a high temperature can be mixed to achieve high temperature distribution in the entire boiler water, thereby making it possible to reduce the risk of corrosion caused by dissolved oxygen.

When the degree of concentration of boiler water is in a medium degree, either one of the water level control by the external water level detection device 50 at the time of low concentration and the water level control by the header water level detection device 60 at the time of high concentration is performed depending on a boiling state of the boiler water.

Then, the water level control in the boiler 10 during the low combustion operation is performed on the basis of the control procedure of FIG. 4. The water level control during the low combustion operation is different from the water level control during the high combustion operation in that, since the boiling of boiler water is weak, the water level is totally raised using the low combustion second electrode 62S and the low combustion first electrode 52S to thereby control the dryness, the overheating degree of the water pipes, and the circulation ratio. The flow of the control is the same as the control procedure of FIGS. 3, and S11, S12, S13, S14, S15 and S16 in FIG. 4 correspond to S1, S2, S3, S4, S5 and S6 in FIG. 3, respectively. Therefore, description of these steps will be omitted.

Embodiment 2

Next, a boiler 10 of Embodiment 2 of the present invention will be described with reference to FIG. 5. Embodiment 2 is different from Embodiment 1 in that a header water level detection device 60 is provided inside an upper header 24. The other configurations of Embodiment 2 are the same as those of Embodiment 1. Therefore, the same components will be denoted by the same reference numerals, and description of these components will be omitted. Hereinbelow, only the different configuration will be described.

Referring to FIG. 5, a water level control tube 61 of the header water level detection device 60 is placed inside the upper header. The water level control tube 61 is a bottomed tubular body. An opening on the upper end of the water level control tube 61 is attached to a top plate 24B of the upper header 24. A communication hole 64 b is formed on a peripheral wall of the water level control tube 61 at a height position near the bottom on the lower end side thereof. The communication hole 64 b is provided for introducing thereinto some of air bubbles in an internal space 24A and liquid droplets generated from air bubbles bursting in the internal space 24A. A single communication hole 64 b or a plurality of communication holes 64 b are provided. Further, the height position in the vertical direction of the bottom of the water level control tube 61 is sufficiently lower than a high combustion second electrode 62M.

Since the water level control tube 61 is attached to the top plate 24B, in order to release gas (air, steam and the like) from the water level control tube 61 when the water level rises inside the water level control tube 61, a single communication hole 64 a or a plurality of communication holes 64 a are provided on the peripheral wall of the water level control tube 61 at a height position near the upper end thereof. Therefore, the inside of the water level control tube 61 communicates with an upper part and a lower part of the inside of the upper header 24 respectively through the upper communication hole 64 a and the lower communication hole 64 b, so that the pressure inside the water level control tube 61 is equal to the pressure inside the internal space 24A.

A low combustion second electrode 62S and a high combustion second electrode 62M in Embodiment 2 have the same configurations as the low combustion second electrode 62S and the high combustion second electrode 62M in Embodiment 1, respectively. Therefore, detailed description thereof will be omitted.

Next, the operation of the boiler 10 of Embodiment 2 will be described. The boiler 10 of Embodiment 2 is controlled on the basis of the control procedures of FIG. 3 and FIG. 4. The operation thereof is substantially the same as the operation of the boiler 10 of Embodiment 1.

Here, a different point from the operation of the boiler 10 of Embodiment 1 will be described. In the boiler 10 of Embodiment 2, air bubbles that have entered the internal space 24A burst inside the internal space 24A, so that an inner wall surface of the internal space 24A and an inner surface of the upper part of each water pipe 23 are covered by films of liquid droplet (water films). At the same time, some of the air bubbles enter the water level control tube 61 through the communication hole 64 b, and liquid droplets generated from burst air bubbles also flow into the water level control tube 61 through the communication hole 64 b. These air bubbles and liquid droplets are detected as the amount of water and appear as the water level inside the water level control tube 61. Therefore, the header water level detection device 60 of Embodiment 2 detects the water level that appears inside the water level control tube 61.

The different point from Embodiment 1 is as described above. Control and the like by a controller 100 are the same as those of Embodiment 1.

According to Embodiment 2, the communication pipes 88 a, 88 b of Embodiment 1 are not required. Further, it is not necessary to provide a space in which the header water level detection device 60 is to be placed separately from the upper header 24. Therefore, it is possible to reduce a placement space compared to Embodiment 1.

Embodiment 3

Next, a boiler 10 of Embodiment 3 of the present invention will be described with reference to FIG. 6 and FIG. 7. A difference from Embodiment 1 is as follows. In Embodiment 1, switching between the external water level control by the external water level detection device 50 and the header water level control by the header water level detection device 60 is automatically performed without determining the degree of concentration of boiler water. On the other hand, in Embodiment 3, switching between the external water level control and the header water level control is performed by determining the degree of concentration of boiler water. The other configurations are the same as those of Embodiment 1. Therefore, the same components will be denoted by the same reference numerals, and description of these components will be omitted. Hereinbelow, only the different configuration will be described.

In Embodiment 3, a header water level detection device 60 is provided with a sixth electrode 62E which detects a high concentration dryness limit water level HE as a header upper limit water level, so that a water supply pump 70 is thereby turned OFF, and a seventh electrode 62F which detects a water level HF as a header lower limit water level which is lower than the high concentration dryness limit water level HE, so that the water supply pump 70 is thereby turned ON. Further, an external water level detection device 50 is provided with an eighth electrode 52G which detects a low concentration dryness limit water level HG as a water level control tube upper limit water level, so that the water supply pump 70 is thereby turned OFF, and a ninth electrode 52J which detects a water level HJ as a water level control tube lower limit water level which is lower than the low concentration dryness limit water level HG, so that the water supply pump 70 is thereby turned ON.

A controller 100 determines the degree of concentration on the basis of whether a combustion time in a burner 30 after discharging boiler water existing inside a boiler body 20 to the outside reaches a combustion time threshold. Then, the external water level control is performed when boiler water is in low concentration, and the header water level control is performed when boiler water is in high concentration. A water level control procedure by the controller 100 is illustrated in FIG. 7.

Next, the operation in Embodiment 3 will be described. Referring to FIG. 7, in S21, the controller 100 measures a combustion time of the burner 30 after full blowing using a built-in timer (not illustrated), and determines whether the measured combustion time reaches a predetermined combustion time (combustion time threshold). When NO is determined, the step proceeds to S22 to control the water level on the basis of the result of detection in the external water level detection device 50. On the other hand, when YES is determined in S21, that is, the detected combustion time has reached the predetermined combustion time (combustion time threshold), the step proceeds to S23 to control the water level on the basis of the result of detection in the header water level detection device 60. Note that the full blowing means discharging the entire boiler water existing inside the boiler body 20 to the outside. Further, instead of performing full blowing, half blowing which discharges an approximately half amount of boiler water to the outside may be performed. Also in this case, the combustion time threshold is set by measuring a combustion time after the half blowing.

More specifically, the degree of concentration of boiler water is low for a while after performing full blowing, even if combustion is performed in the burner 30. When the degree of concentration of boiler water is low (in the case of low concentration), the number of generated air bubbles is smaller than that when the degree of concentration of boiler water is high (in the case of high concentration). In other words, the behavior of boiler water is relatively stable. Therefore, even when control based on the result of detection in the external water level detection device 50 is performed, a desired dryness of steam can be obtained. However, as the combustion time of the burner 30 is accumulated after performing the full blowing, the degree of concentration of boiler water increases. When the degree of concentration increases until the electric conductivity of boiler water reaches a predetermine value, the behavior of boiler water rapidly changes. Accordingly, bubbling gets more active due to an increase in the viscosity of boiler water, the number of generated air bubbles increases, and the diameter of splashed liquid droplets when air bubbles burst increases. As a result, the dryness is reduced.

Therefore, in the water level detection by the external water level detection device 50, the combustion time threshold is associated with a state in which water behavior change of a degree by which a desired dryness cannot be obtained occurs. Time measurement is started at the timing of starting combustion by the burner 30 after performing full blowing. Water level detection is performed in the external water level detection device 50 until the measured time reaches the combustion time threshold, and the water level of boiler water is controlled on the basis of the result of the detection. On the other hand, when the measured time has reached the combustion time threshold, water level detection is performed by measuring the water level inside the water level control tube 61 of the header water level detection device 60, and the water level of boiler water is controlled on the basis of the result of the detection.

In this specification, “the measured time reaches the combustion time threshold” may include both of the case where the measured time becomes equal to or more than the combustion time threshold and the case where the measured time becomes more than the combustion time threshold. Further, “the measured time does not reach the combustion time threshold” may include both of the case where the measured time is equal to or less than the combustion time threshold and the case where the measured time is less than the combustion time threshold.

The control based on the result of detection in the external water level detection device 50 is performed so that the water level inside a water level control tube 51 is located between the eighth electrode 52G and the ninth electrode 52J. Further, the control based on the result of detection in the header water level detection device 60 is performed so that the water level inside the water level control tube 61 is located between the sixth electrode 62E and the seventh electrode 62F.

Embodiment 4

Next, a boiler 10 of Embodiment 4 of the present invention will be described with reference to FIG. 8 and FIG. 9. A difference from Embodiment 3 is as follows. In Embodiment 3, determination of the degree of concentration of boiler water is performed on the basis of the combustion time. On the other hand, in Embodiment 4, the degree of concentration of boiler water is determined on the basis of the electric conductivity of the boiler water. The other configurations are the same as those of Embodiment 3. Therefore, the same components will be denoted by the same reference numerals, and description of these components will be omitted. Hereinbelow, only the different configuration will be described.

In Embodiment 4, as illustrated in FIG. 8, there is provided an electric conductivity measuring sensor 90 which measures the electric conductivity of boiler water inside a downcast pipe 84. Further, by a water level control procedure illustrated in FIG. 9 performed by a controller 100, the water level is controlled on the basis of the result of detection in an external water level detection device 50 when the electric conductivity does not reach an electric conductivity threshold. Further, when the electric conductivity measured by the electric conductivity measuring sensor 90 becomes equal to or higher than the electric conductivity threshold, the water level is controlled on the basis of the result of detection in a header water level detection device 60. The electric conductivity threshold is configured so as to correspond to a predetermined degree of concentration.

The operation of Embodiment 4 will be described. Referring to FIG. 9, in S31, it is determined whether the electric conductivity measured by the electric conductivity measuring sensor 90 reaches the electric conductivity threshold. When NO is determined in S31, the step proceeds to S32 to perform control on the basis of the result of detection in the external water level detection device 50. When the measured electric conductivity is lower than the electric conductivity threshold, the degree of concentration of boiler water is low. Therefore, the behavior of boiler water is relatively stable. Therefore, even when control based on the result of detection in the external water level detection device 50 is performed, a desired dryness of steam can be obtained.

When the degree of concentration increases until the electric conductivity measured by the electric conductivity measuring sensor 90 becomes equal to or higher than the electric conductivity threshold, YES is determined in S31, and the step proceeds to S33 to perform detection of the water level inside the tube in the header water level detection device 60. Then, the water level inside the tube is controlled on the basis of the result of the detection.

When the degree of concentration increases until the electric conductivity becomes equal to or higher than the electric conductivity threshold, the behavior of boiler water rapidly changes. Accordingly, bubbling gets more active due to an increase in the viscosity of boiler water, and the diameter of splashed liquid droplets when the air bubbles burst increases. As a result, the dryness is reduced.

Therefore, in the water level detection by the external water level detection device 50, the electric conductivity threshold is associated with a state in which water behavior change of a degree by which a desired dryness cannot be obtained occurs. Water level detection is performed in the external water level detection device 50 until the electric conductivity measured by the electric conductivity measuring sensor 90 reaches the electric conductivity threshold, and the water level of boiler water is controlled on the basis of the result of the detection. On the other hand, when the electric conductivity measured by the electric conductivity measuring sensor 90 becomes equal to or higher than the electric conductivity threshold, the water level inside the tube is detected in the header water level detection device 60, and the water level inside the tube is controlled on the basis of the result of the detection.

In this specification, “the measured electric conductivity reaches the electric conductivity threshold” may include both of the case where the measured electric conductivity becomes equal to or higher than the electric conductivity threshold and the case where the measured electric conductivity becomes higher than the electric conductivity threshold. Further, “the measured electric conductivity does not reach the electric conductivity threshold” may include both of the case where the measured electric conductivity is equal to or lower than the electric conductivity threshold and the case where the measured electric conductivity is lower than the electric conductivity threshold.

Embodiment 5

Next, a boiler 10 of Embodiment 5 of the present invention will be described with reference to FIGS. 10 and 11. A difference in Embodiment 5 from Embodiment 1 is a dryness maintaining control procedure and a circulation ratio ensuring control procedure, that is, the control procedures of FIG. 3 and FIG. 4 are changed to control procedures of FIG. 10 and FIG. 11, respectively. More specifically, in S1 of FIGS. 3 and S11 of FIG. 4, when the high combustion first electrode 52M and the high combustion second electrode 62M detect the non-existence of water, the water supply pump 70 is immediately turned ON. However, in S41 of FIG. 10 and S51 of FIG. 11, the water supply pump 70 is turned ON when set time elapses after both of the high combustion first electrode 52M and the high combustion second electrode 62M detect the non-existence of water.

Further, in S3 of FIGS. 3 and S13 of FIG. 4, when the high combustion first electrode 52M and the high combustion second electrode 62M detect the existence of water, the water supply pump 70 is immediately turned OFF. However, in S43 of FIG. 10 and S53 of FIG. 11, the water supply pump 70 is turned OFF when respective different set times elapse after either one of the high combustion first electrode 52M or the high combustion second electrode 62M detects the existence of water. The other configurations are the same as those of Embodiment 1. Therefore, the same configurations will be denoted by the same reference numerals, and description thereof will be omitted.

In Embodiment 5, each of the set times is adjusted depending on the pressure inside the boiler body 20 (detected by a sensor (not illustrated) which detects the pressure of steam inside the boiler body 20) and the temperature of supplied water (detected by a sensor (not illustrated) which detects the temperature of supplied water inside the water supply pipe 80). Specifically, each of the set times is made shorter as the pressure inside the boiler body 20 increases or the temperature of supplied water drops.

Embodiment 6

Next, a boiler 10 of Embodiment 6 of the present invention will be described with reference to FIG. 12. The present invention can be applied not only to a boiler having a boiler body structure in which a plurality of water pipes 23 are annularly arranged as in Embodiments 1 to 5, but also a boiler having a structure in which a plurality of water pipes 23 are arranged to form a rectangular parallelepiped shape as the boiler 10 illustrated in FIG. 12. In the following description for Embodiment 6, configurations different from those of Embodiment 1 will be mainly described. The other components corresponding to those in Embodiment 1 will be denoted by the same reference numerals, and description of these components will be omitted.

In view of space saving, the boiler 10 illustrated in FIG. 11 is provided with a boiler body 20 which includes a rectangular parallelepiped lower header 22, a rectangular parallelepiped upper header 24, and a water pipe group which includes a plurality of water pipes 23 vertically arranged in a standing manner between the headers 22 and 24. In the water pipe group of the boiler body 20, among water pipes 23 arranged on both outer sides in the longitudinal direction, adjacent water pipes 23 are connected to each other through a coupling member (not illustrated) to form a pair of water pipe walls (not illustrated). Therefore, the boiler body 20 is provided with a rectangular parallelepiped compartment formed by the lower header 22, the upper header 24, and the pair of water pipe walls. The compartment corresponds to the combustion chamber 21A in Embodiments 1 to 5. In the compartment, flame from a burner 30 which is provided on one end in the longitudinal direction performs combustion reaction and, at the same time, flows toward an exhaust gas outlet port 103.

In the boiler 10 illustrated in FIG. 11, although not illustrated, a downcast pipe 84 which has the same configuration as that of Embodiments 1 to 5 is provided. More specifically, the upper header 24 in which concentrated boiler water is retained and the lower header 22 to which new boiler feed water is supplied are connected to each other through the downcast pipe 84 to thereby allow boiler water inside the upper header 24 to naturally circulate. Since the operation of the boiler 10 illustrated in FIG. 12 is substantially the same as the operation in Embodiments 1 to 5, detailed description thereof will be omitted.

Further, as with the configuration in each of Embodiments 1 to 5, an external water level detection device 50 communicates with an internal space 24A of the upper header 24 and an internal space 22A of the lower header 22 respectively through a communication pipe 87 a and a communication pipe 87 b. Further, a header water level detection device 60 communicates with the internal space 24A of the upper header 24 through communication pipes 88 a, 88 b.

The present invention is not limited to Embodiments 1 to 6 described above, and includes a boiler of the following modes. In each of Embodiments 1 to 5, the boiler 10 is a separatorless boiler. However, the boiler may be provided with a small separator which separates water droplets (liquid droplets) from steam by centrifugation. Further, in Embodiment 1, in S1 of FIG. 3, when non-existence of water is detected, the step immediately proceeds to S2 to turn ON the water supply pump 70. However, the water supply pump 70 may be turned ON after the elapse of a predetermined delay time from the detection of the non-existence of water. Further, in S3 of FIG. 3, when the existence of water is detected, the step immediately proceeds to S3 to turn OFF the water supply pump 70. However, the water supply pump 70 may be turned OFF after the elapse of a predetermined delay time from the detection of the existence of water. When controlling the water supply pump 70 with the delay time in this manner, since water levels detected by the electrodes and various set water levels are different from each other, various set water levels are estimated water levels after the delay time has elapsed. 

1. A boiler comprising: a boiler body including an upper header, a lower header, and a large number of water pipes to be heated by a burner, the water pipes connecting the upper header and the lower header to each other, wherein the water level in the water pipes when combustion by the burner is stopped is lower than the upper ends of the water pipes; water supply means supplying boiler feed water into the boiler body; control means controlling the operation of the water supply means depending on the water level in the boiler body; header water level detection means detecting a water level existing inside the upper header by boiler water inside the water pipes being pushed up by air bubbles generated by boiling of the boiler water; and downcast pipe allowing a lower part of the upper header and the lower header to communicate with each other, wherein the control means performs first control for controlling the operation of the water supply means so as to lower a boiler body water level when the water level detected by the header water level detection means reaches a header set water level and second control for controlling the operation of the water supply means so as to raise the boiler body water level when the amount of drop in the boiler body water level reaches a predetermined amount after the first control.
 2. The boiler according to claim 1, wherein the predetermined amount is adjusted depending on any one or more of the pressure inside the boiler body, the temperature of supplied water, and the degree of concentration of boiler water.
 3. The boiler according to claim 1, further comprising external water level detection means placed outside the boiler body, the external water level detection means communicating with an internal space of the upper header and an internal space of the lower header through communication pipes and having an electrode detecting a boiler body external water level, wherein determination of the predetermined amount by the control means is performed using the electrode.
 4. The boiler according to claim 3, wherein determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water.
 5. The boiler according to claim 1, wherein the header water level detection means includes a first electrode detecting the header set water level and a second electrode detecting a water level lower than the header set water level, and determination of the predetermined amount by the control means is performed using the second electrode.
 6. The boiler according to claim 5, wherein determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the second electrode detects the non-existence of water.
 7. The boiler according to claim 1, wherein the header water level detection means includes an electrode detecting the header set water level, and determination of the predetermined amount by the control means is performed on the basis of the elapse of set time after the electrode detects the non-existence of water. 